1. Field of the invention
The present invention relates generally to a method of marking with a laser beam, and more particularly, to a method suitable for marking of characters, numerals, signs, codes, figures or their combinations representing identification information and the other information on products or articles such as glass, plastics and products including as their parts glass, plastics or the like (for example, a liquid crystal display panel, a plasma display panel, etc. including a glass substrate, or a cathode ray tube) (the products include parts or components), or on members or materials constituting the products (particularly, a transparent or translucent member such as a glass substrate) in a process of manufacturing or fabricating of the products, or in a preceding stage of manufacture.
2. Description of the Prior Art
For management of manufacturing processes, management of manufactured products or members used for the production, and the other management, characters, numerals, signs, codes, figures and the like representing identification information including the date of production, the serial number and so on are marked on products (which have not been completed yet as parts, components, products or articles, or the like in many cases) or on members or materials composing the products in the manufacturing process or in the preceding stage. A special marking method is employed for glass products, products including glass as their parts, or members composing the products (a glass substrate, plate or base, etc.).
For example, JP-A-5-309552 discloses a method of forming small notches representing a binary code on an end face of a glass substrate of a liquid crystal panel (an object to be marked), and a method of directly engraving characters, numerals, signs and the like on the surface of a side edge of a glass substrate with a laser beam.
These methods have some problems. For example, the mark is difficult to read because the glass substrate is transparent. Glass powder is produced in the marking process, and is difficult to remove. The glass substrate is liable to be cracked when the subsequent process includes heat treatment since it has been notched or engraved.
The other marking method is disclosed in JP-A-60224588. This method is a method of placing a metal under a transparent member (an object to be marked) such that it is brought into close contact with the bottom surface of the transparent member, and irradiating the metal with a YAG laser beam through the transparent member from above the transparent member such that the laser beam is focused on the bottom surface of the transparent member. Such control is performed that a gap of less than 10 .mu.m exists between the transparent member and the metal. Characters and the like are marked on the bottom surface of the transparent member with spatter of the metal by i radiation of the focused laser beam.
In this method, the object to be marked is significantly limited. For example, the method is not applicable to an object which does not transmit or does not easily transmit the laser beam. In the case of an object having another material existing on its surface, for example, coated with an SiO.sub.2 film, it is difficult to focus the laser beam on the bottom surface c f the object. The method is not applicable to an object through which the metal is not easily irradiated or cannot be irradiated with the laser beam (for example, a cathode ray tube).